If you do not wish to synchronise your iPad and iPhone messages, on your iPad*you will need to first disabled iCloud Messages:
Settings > [Your Name / AppleID] > iCloud > [Apps Using iCloud] Show All > Messages > Sync This iPad - set to OFF
(*) You can also disable iCloud Messages on your iPhone if you don't wish to keep a copy of your Messages in iCloud. If iCloud Messages is disabled, your Messages will instead be included within your iCloud Backup - if enabled.
Having disabled iCloud Messages on your iPad, you can delete messages from your iPad without affecting messages stored on your iPhone.
If you don't wish to see any messages forwarded from your iPhone to your iPad, you can disable Message Forwarding from your iPhone...:
On your iPhone:
Settings > Messages > Text Message Forwarding - set to OFF - and/or explicitly disable your iPad from the list of devices.